1. Field of the Invention
The present invention relates to a signal processing device, and, in particular, to a signal processing device excellent for use in a recording/reproducing apparatus for recording and reproducing a signal at a high density.
2. Description of the Related Art
With the advancement of digital magnetic recording and reproducing technologies in recent years, digital VTRs have been widely developed. Just as in conventional analog VTRs, in digital VTRs, tracking control needs to be carried out during signal reproduction, so that various tracking control techniques have been proposed.
Of these tracking control techniques, the technique which has drawn attention is the one that superimposes a predetermined pilot signal component during modulation of a digital signal data row to be recorded, and employs this pilot signal during signal reproduction to perform tracking control.
FIG. 1 is a schematic view of an arrangement of a recording system of a digital VTR employing this type of technique. In the figure, ch1 and ch2 each denote a rotating head, mounted to a rotating drum at a phase difference of 180 degrees, for recording modulated digital signals onto a magnetic tape which is a recording medium. T denotes a magnetic tape.
A description will now be given of the operation.
A video signal, input from a terminal 101, is supplied to a digital recording signal processing circuit 102 that encodes the video signal with high efficiency, and further performs error correction encoding to form a digital data row in accordance with a recording data format along with audio data and other supplementary data.
This digital data row is supplied to a digital modulating/pilot adding circuit 103 that modulates the digital data row supplied from the processing circuit 102, this digital data row modulation giving redundancy to data, subjected to, for example, 24-25 conversion. The redundancy is utilized to add a pilot signal component.
More specifically, for example, bit values of "1" and a "0" are added to the beginnings of 25-bit data with every 24-bit data, each of which resulting 25-bit data are output as bit streams. These bit streams are each subjected to NRZI modulation. A DC component, an f1 component (first pilot signal frequency), and f2 component (second pilot signal frequency) are extracted from each of the two types of NRZI-modulated bit streams to calculate the total sum of the components of each bit stream. Each total sum is added to a past total sum cumulative value to form a cumulative value for each bit stream. These cumulative values of the two types of bit streams are compared and the 25-bit bit stream making the cumulative value smaller is selected for output. In this case, the bit stream to be output is a bit stream whose DC, f1, and f2 components are suppressed.
Here, when a predetermined pattern signal is superimposed on (for example, subtracted from) each extracted DC component, the aforementioned bit streams possess frequency components with respect to the aforementioned pattern signals. With the frequencies of the predetermined pattern signals set at desired values of f1 and f2, the desired pilot signal frequency components are superimposed onto the modulated digital bit streams.
For example, if the bit rate of each bit stream is defined as fb, the frequency, f1, of the first pilot signal defined as fb/90, and the frequency, f2, of the second pilot signal defined as fb/120, the repeating pattern signal at a 90-bit cycle or a 120-bit cycle is subtracted from each DC component of the two types of bit streams.
The thus-obtained modulated digital bit streams are supplied to a switching circuit 104 as modulated digital recording signals, which are supplied alternately to the rotating heads ch1 and ch2 by a head switching pulse (HSW) formed in accordance with the rotating phase of each rotating head ch1, ch2. The heads ch1 and ch2 rotate at a phase difference of 180 degrees and alternately trace the magnetic tape, T, so as to successively form a large number of helical tracks parallel to one another which are used to record the aforementioned modulated digital recording signals.
FIG. 2 illustrates a recording pattern on the magnetic tape, T. As illustrated in the figure, pilot signals are superimposed on every other track of the large number of helical tracks formed, so that the pilot signal with the frequency of f1 and the pilot signal with the frequency of f2 are alternately superimposed at a 4-track cycle. In the formation of such a recording pattern, for example, the pattern signals having frequencies of f1 and f2 are alternately subtracted from the aforementioned DC component during recording by the head ch1, whereas this is not performed during recording by the head ch2.
FIG. 3 is a block diagram illustrating an arrangement of a conventional reproducing system for reproducing the signal recorded in the format illustrated in FIG. 2.
The modulated signals, alternately reproduced by the heads ch1 and ch2, are input to a head switching circuit 109 via reproduction amplifiers 107 and 108, respectively. The circuit 109 is switched by the HSW generated from a drum rotation detection circuit 115 so as to produce a continuous reproduced signal which is input to a digital signal reproducing processing circuit 110, and an f1 detection circuit 112 and an f2 detection circuit 113. The digital signal reproducing processing circuit 110 operates to perform digital data row modulation, error correction, highly-efficient encoding, etc. to output reproduced information data (video data) to an output terminal 111.
The f1 detection circuit 112 and the f2 detection circuit 113, each containing, for example, an analog bandpass filter, extract pilot signal components that are supplied to a tracking control circuit 116. After detection of the level of the aforementioned detection circuits 112 and 113, the tracking control circuit 116 takes the difference between the detected level outputs. If tracking control is performed such that the head ch2 reproduces a self-recording track, the head ch2 traces a track that does not have a pilot signal superimposed thereon, which means that the f1 and f2 components are obtained from both adjacent tracks, respectively. Here, the difference between the pilot signal components can be used to obtain a signal that indicates tracking error of the head ch2. It is to be noted that since a tracking error signal cannot be obtained during tracing by the head ch1, the tracking control circuit 116 samples and holds the tracking error signal obtained just before the tracing. Since a tracking error signal reverses in polarity every 2-track cycles, the tracking control circuit synchronizes with the HSW to appropriately reverse the sign of the aforementioned difference value.
The thus-obtained tracking control signals are supplied to a capstan control circuit 117. The transportation of the magnetic tape T is controlled so that the heads trace the desired tracks.
A description will now be given of an analog detection circuit employed as f1 detection circuit and f2 detection circuit, with reference to FIGS. 4 and 5. FIG. 4 illustrates a construction of a conventional common analog detection circuit, wherein an analog signal to be detected is input to an input terminal 121. FIG. 5A illustrates a spectrum of the analog signal input to the input terminal 121.
For simplification, FIG. 5A characteristically illustrates the pilot signal components alone. Although not particularly illustrated, it is obvious that the spectrum of each modulated digital bit stream is also superimposed.
The input signal is supplied to a bandpass filter (BPF) 122 that extracts only the f1(f2) component. In FIG. 5A, c1 and c2 represent the characteristics of the BPF to extract the f1 and f2 components, respectively. In the following description, a circuit that extracts the f1 component is taken as an example. The BPF 122 extracts the f1 component alone in accordance with the aforementioned characteristic c1, and inputs the f1 component to an absolute value detection circuit (ABS) 123.
FIG. 5B illustrates a spectrum distribution of the output signal of the ABS 123. As illustrated in the figure, a high harmonic signal, having a frequency an even number of times greater than the f1 frequency, is developed. Accordingly, if the output signal from the ABS 123 remains in the form of an absolute value, the high harmonic component remains. Therefore, this high harmonic component is removed by a lowpass filter (LPF) having such a characteristic, indicated by C3 of FIG. 5, as to extract only the DC component as illustrated in FIG. 5D. It is obvious that the circuit for extracting the f2 component is constructed in the same way.
As is apparent from the foregoing description, at the recording side circuit of such a conventional digital VTR, the 25-bit data with a leading bit value of "1" and that with a starting bit value of "0" each require circuits that extract from the bit stream, during digital processing, a DC component, an f1 component, and an f2 component, respectively, as well as a cumulative value calculating circuit. In addition, circuits are required for extracting an f1 component and an f2 component, respectively, by analog processing, during reproduction.
Accordingly, in such a VTR, a digital signal processing section and an analog signal processing section must be provided for tracking control alone, which results in a larger number of hardware components. More specifically, a large space is required for accommodating a special analog circuit which must be externally attached in the digital VTR constructed mostly of digital processing circuits. In addition, it is necessary to provide a lowpass filter to remove the high harmonic component, developed by the detection circuit disposed behind the bandpass filter, thereby making it difficult to achieve an efficient circuit arrangement.